The present invention is directed to an electrophotographic printing or copying means having a developer region for developing an image on a charge image. The present invention is more specifically directed to such a printing or copying means using a developer mix of toner particles and carrier particles.
In electrophotographic printing or copying machines, a latent charge image is generally generated on a photoconductor by means of an optical character generator which can, for example, be an LED character generator, and this charge image is then tinted with toner particles in a developer station. The black and also colored toner particles are charged electrostatically and are applied continuously to the photoconductor in a developer gap. The, for example, positively charged toner is attracted by the exposed places on the charge image, whilst the unexposed positively charged surfaces repel it.
When "negative toner" is used, the photoconductor layer is likewise negatively charged and is exposed with the characters to be developed. Both uses are referred to as reversal development.
So that the toner particles adhere to the charge image, they are triboelectrically charged. This triboelectric charging takes place primarily triboelectrically as a result of the friction of the particles on one another in the developer station. The quality of the subsequent character image depends essentially on this interaction of electrostatic and Van der Waals forces. The development process, including the toner feed and regulation with the toner materials used as well as the constructive design of the developer station are therefore some of the most complex structures in an electrophotographic printing or copying machine.
The developer mixture used most frequently is a two-component mixture of toner and of ferromagnetic carrier particles. The two components are intermixed in the developer station and are applied to the photoconductor in the developer gap via a developer roller designed as a magnetic roller. In view of the special properties of the toner and of the carrier material, the mixing operation brings about a charging of the toner which is caused by frictional electricity (triboelectricity). The development operation can be assisted by a bias voltage additionally applied in the region of the developer gap.
Developer stations of the type mentioned are known, for example, from U.S. Pat. No. 5,023,664 or U.S. Pat. No. 4,952,279.
Furthermore, EP-A-0,430,098 and GB-A-2,226,156 describe developer stations having a developer chamber with developer rollers arranged therein, via which developer mixture having the printing-toner concentration is fed to a developer gap. Moreover, the developer stations contain a mixing chamber, in which excess developer is mixed with fresh toner, and a developer-station sump having a mixer which is arranged therein and via which depleted developer mixture and enriched developer mixture are mixed to form a developer mixture having the printing-toner concentration.
When developer stations of this type are used in electrophotographic printing machines, in which images of widely varying image density and composition have to be tinted in any sequence, a problem which arises during the tinting, inter alia on account of the sharply fluctuating toner consumption, is the insufficient long-term stability of the mixing-in of the toner and the fluctuating triboelectricity. Where electrophotographic printing machines are concerned, the degree of tinting of the charge image is determined, for example by means of a test toner mark on the photoconductor, and fresh toner is fed to the developer station in dependence on the toner consumption. However, the mixing-in of fresh toner impairs the triboelectric charging of the developer mixture. A further problem is the sharply fluctuating toner consumption, for example when large image surfaces suddenly have to be tinted. This can lead to a sudden depletion of the toner concentration in the developer station, before the toner concentration is then increased again and stabilized by the supply of fresh toner.